For many structural applications fiber gratings have been proposed as a method to measure strain. A great deal of work has been done investigating the usage of fiber gratings to measure axial strain and temperature. In many cases that involve embedment into a structure, the fiber grating is subject to transverse strain that may result in a spectral shift on the order of the spectral shifts resulting from longitudinal strain and or temperature shifts. E. Udd in U.S. Pat. No. 5,591,965, Jan. 7, 1997 describes a three axis strain and temperature fiber grating sensor formed by writing two overlaid fiber gratings onto birefringent fiber. As an example wavelengths of the fiber gratings can be written at 1.300 and 1.550 microns. By writing onto the highly birefringent fiber four gratings are established. In the case of about a two millimeter beat length at 630 nm they would be at 1300.0, 1300.6, 1550.0 and 1550.8 nm. The birefringent axes are well defined so that transverse strain can be measured along with longitudinal strain and temperature through four equations in four unknowns.
Further improvement in the measurement of transverse strain were made by E. Udd in the patent application Ser. No. 08/707,861, xe2x80x9cTransverse Strain Measurements Using Fiber Optic Grating Based Sensorsxe2x80x9d, filed Sep. 9, 1996.
For many applications it is desirable to be able to quickly and efficiently measure subsets of three axes of strain and temperature. An area that has been investigated closely is the measurement of longitudinal strain and temperature. M. G. Xu, H. Geiger and J. P. Dakin in xe2x80x9cMultiplexed Point and Stepwise Continuous Fibre Grating Based Sensors: Practical Sensor for Structural Monitoring?xe2x80x9d, Proceedings of SPIE, Vol. 2294, p. 94, 1994 describe the usage of dual overlaid fiber gratings to measure strain and temperature. The major difficulty with this approach is that to obtain reasonable accuracy using conventional single mode fiber widely separated wavelengths must be used. In the case of the paper by Xu et. al 850 and 1300 nm edge light emitting diodes were used. These two wavelengths are far enough apart to have modest strain and temperature resolution but problems associated with bend loss and high attenuation in conventional fibers have severely limited the utility of this approach. What is needed is an approach that measures strain and temperature accurately using wavelengths that result in fibers supporting low loss and high bend resistance.
It is also important to be able to process information from multiparameter fiber grating sensors quickly and accurately. For many applications, notably including aircraft, missiles and spacecraft it is necessary to respond quickly and accurately to high speed events. Extremely high speed events are also of interest to perform diagnostics during ballistic tests, rocket motor firing and explosions. To support these measurements very high speed demodulation systems are required in some cases with frequency response on the order of 10s of MHz. In order to meet cost and performance goals it is also necessary to multiplex these devices.
High speed demodulation methods that allow multiparameter sensing to be accomplished while multiplexing significant numbers of fiber sensors along a single fiber are needed for these applications. Current demodulator systems are designed for modest speeds. Typical performance of fiber etalon based systems that are currently marketed by Research International and Micron Optics run at 50 to 200 Hz with sufficient resolution to support multiparameter sensing. Higher speed, single channel fiber grating demodulation systems are commercially available from Electrophotonics and Blue Road Research. The current models run at 5 to 7 kHz and are designed to monitor one single element fiber grating sensor severely limiting their ability to support multiparameter distributed sensing.
Systems are needed that support multiparameter sensing at much higher speeds while enabling multiplexing of fiber grating sensors along a single fiber line in significant numbers.
In the present invention high speed demodulation systems are described for supporting one or more fiber grating sensors that are subject to temperature, longitudinal strain, transverse strain or other environmental parameters that result in a change in their spectral response. This invention improves the accuracy and speed of measurements made on fiber grating sensors formed on birefringent optical fiber by identifying the minimal number of spectral peak positions that must be measured to insure an accurate result. For the case of dual wavelength fiber gratings written onto birefringent fiber this involves selecting two or three of the four possible spectral peaks to process to allow rapid measurement of axial strain and temperature, or pressure, or three axes of strain. Very high speed demodulation and multiplexed fiber grating sensor systems can be supported by employing ratiometric techniques that use chirped fiber gratings, overcoupled couplers or Mach-Zehnder or Michelson interferometers in combination with appropriately placed fiber grating filters. By combining these techniques it is possible to support very high speed multiparameter sensing using fiber gratings.
Therefore it is an object of the invention to provide high speed demodulation systems for fiber grating sensor systems.
Another object of the invention is to provide demodulation systems that are capable of supporting multiparameter sensing at higher speed.
Another object of the invention is to provide means to multiplex large numbers of fiber sensors for multiparameter and high speed sensing.
Another object of the invention is to provide higher speed performance while retaining the accuracy of measurements.